DME is a ground-based navigation system which consists of a network of ground transponders and airborne interrogating units (interrogators). The main purpose of DME transponder operations is to allow aircraft to identify and obtain a range to a DME transponder. In operation, an interrogator transmits DME pulse pair signals to be received by an intended ground transponder on a predetermined downlink frequency within the DME frequency band of 962 MHz to 1150 MHz. Upon receiving an interrogation pulse pair signal the ground transponder determines whether the received signal is a valid interrogation signal by checking the spacing between the two pulses in the DME pulse pair signal. If a valid interrogation is detected, ground transponder transmits a reply signal on a predetermined uplink frequency after a preset delay of approximately 50 μs. The reply signal consists of a pulse pair with a fixed spacing that is transmitted on a different predetermined uplink frequency within the DME frequency band. The specific pairing of interrogation and replying frequencies and the spacing between the pulses in the interrogation and replying pulse pair signals defines the DME channel/mode of the DME operation.
The interrogation and replying operation between an interrogator (e.g., aircraft) and a ground transponder enables the aircraft to determine a range to the transponder based on the observed round-trip delay between the transmission of the interrogation signal and receipt of the reply signal. FIGS. 1 and 2 illustrate the operating principles of legacy DME equipment using the interrogation and reply method of operation.
There are 126 frequency pairings (Channel #001˜#126) and four spacing pairings (Mode X W Y Z) allocated for DME operation within the DME frequency band. Each channel consists of an interrogation frequency band and a replying frequency band that are separated from adjacent bands by 1 MHz. The purpose of defining DME channels and modes is to minimize the co-channel interference between adjacent DME transponders. It is important that adjacent DME transponders operate either on a different frequency or use different modes when operating on the same uplink or down link frequency.
Since the DME frequency range includes the uplink and downlink Secondary Surveillance Radar (SSR) frequency bands, the DME channels that are within these SSR frequency bands need to be reserved from usage for sites whose operating coverage area (including both interrogation and replying) overlaps with the coverage area of an operating SSR. The FAA Next Generation (NextGen) Automatic Dependent Surveillance—Broadcast (ADS-B) surveillance system, which is largely built upon SSR links, includes DME channels that overlap the SSR frequencies and these overlapping DME channels cannot be assigned to any DME operations.
Interrogation signals consist of pseudo-randomly spaced DME pulse pairs. The interrogation and reply pulses are modulated at different frequencies to minimize interference.
An interrogation signal containing pseudo-randomly spaced DME pulse pairs is transmitted by the DME interrogator on a DME downlink frequency to the DME transponder, as shown in FIG. 1. Upon receiving the interrogation signal, the DME transponder determines whether the pulse pair of the interrogation signal is valid and when the received interrogation signal is valid the DME transponder replies with a reply signal containing an identical DME pulse pair to the interrogator on a DME uplink frequency after a fixed transponder delay. The DME interrogator receives the reply signal and correlates the received pulse pair in the reply signal with the known pulse pair transmitted in the interrogation signal to determine the total delay time. By subtracting the known transponder delay time (td) from the total delay time, dividing the resulting time delay by two, and then multiplying the result by the speed of light, the DME interrogator determines the range from the DME interrogator to the DME transponder.
The DME interrogation signals containing pseudo-randomly spaced DME pulse pairs do not carry any information other than the unique randomness that is only meaningful to the DME interrogator. To distinguish the pseudo-randomly spaced DME pulse pairs of a DME interrogator's interrogation signal from other interrogation signals from other DME interrogators, the pseudo-randomly spaced sequence of pulse pairs are known only to the DME interrogator so that when the DME interrogator receives a reply signal, the DME interrogator performs a correlation between the transmitted DME interrogation signal and the received DME reply signal to determine if the correct pseudo-randomly spaced DME pulse pairs can be identified in the received reply signal. An example of the pseudo-randomly spaced sequence of pulse pairs for a DME interrogation signal and the DME reply signal are shown in FIG. 2. The randomness of the interrogation pulse pair sequence varies from DME interrogator to DME interrogator. For simplicity, DME interrogators often use a random pick of a set of preselected spacing between two pairs of pulses to “stagger” the interrogation pulse pairs rather than arranging the pulse pair positions using truly random positions.
While the main purpose of the DME transponder is to reply to the interrogation signals from aircraft, the DME transponder also broadcasts its identity periodically. In accordance with international standards, approximately every 40 seconds, each transponder broadcasts its station ID using International Morse code in a time period not exceeding 10 seconds. To transmit the station ID, the DME transponder transmits a Morse code dot as a 0.1 to 0.16 second period consisting of pulse pair signals with a fixed rate of 1350 pulse pair per second (pp/s) and a Morse code dash has a period that is three times longer than the Morse code dot.
When there are either no interrogations or very few interrogations, a DME transponder maintains a minimum pulse pair transmission rate of 700 pp/s by randomly transmitting pulses that are not in response to an interrogation. When there are too many interrogations the DME transponder omits some of the interrogations and maintains a maximum transmission rate of between 2610 and 2790 pp/s. Based on the “transponder recovery time”, two DME pulses that are received above a minimum triggering level no closer than 8 μs shall be able to be recognized by the transponder for decoding processing.
After receiving a DME interrogation signal containing pseudo-randomly spaced DME pulse pairs that the DME transponder determines is valid, the DME transponder will not respond to any new interrogation signals for up to 60 μs. During this “transponder dead time”, the DME transponder will not reply to a second DME interrogation signal if the second DME interrogation signal arrives within 60 μs of the arrival time of the first DME interrogation signal that the DME transponder determines is valid. The purpose of this “transponder dead time” is to suppress unwanted DME interrogations caused by echo or multipath signals. The result of this “transponder dead time” is that no two DME reply signals will be transmitted closer than 60 μs on the DME reply signal frequency due to the “transponder dead time” period.
The next generation (NextGen) national airspace system (NAS) relies on GNSS-based surveillance systems (i.e., GPS) to provide aircraft position information both on the ground and airborne to the ground for surveillance and control purposes. However, existing GNSS-based surveillance systems can be disrupted by solar storms that cause severe ionosphere delay variations that degrade both GPS and WAAS and affect L1 and L5. Current correction broadcasting cannot keep up with the rapid variations in times of solar storms. In addition, there are practical concerns regarding intentional and unintentional interference, regional and temporal unavailability of GPS services, thereby causing a severe degradation or loss of GNSS-based surveillance capability.
What is needed is a system and method that provides a ground surveillance capability for determining aircraft position in the NAS (National Airspace System) as a backup to or to augment the existing GNSS-based surveillance systems.